Are you using the same keystore file when you first uploaded to google play?
Even if you’re just uploading a “test” version of the app you don’t release to any track, I believe that it will still save the certificate that the app was signed with.
If you did generate a new keystore or change the specific keystore that you’re signing with, your private signing key might have been saved in google play, so if it changed you can request a reset:
That way your new signing key would be the one that google expects.
If you DIDN’T generate a new keystore, I’m not entirely sure what could be the issue 